Soil-plant relationships and contamination by trace elements: A review of twenty years of experimentation and monitoring after the Aznalcóllar (SW Spain) mine accident

Alterations in the Anatomy and Ultrastructure of Leaf Blade in Norway Maple (Acer platanoides L.) Growing on Mining Sludge: Prospects of Using This Tree Species for Phytoremediation

Alterations in leaf architecture can be used as an indicator of the substrate toxicity level as well as the potential of a given plant species in the phytoremediation of polluted areas, e.g., mining sludge. In this work, we demonstrated, for the first time, the nature and scale of alterations in leaf architecture at the tissue and cellular levels occurring in Norway maple growing on mining sludge originating from a copper mine in Lubin (Poland). The substrate differs from other mine wastes, e.g., calamine or serpentine soils, due to an extremely high level of arsenic (As). Alterations in leaf anatomy predominantly included the following: (1) a significant increase in upper epidermis thickness; (2) a significant decrease in palisade parenchyma width; (3) more compact leaf tissue organization; (4) the occurrence of two to three cell layers in palisade parenchyma in contrast to one in the control; (5) a significantly smaller size of cells building palisade parenchyma. At the cellular level, the alterations included mainly the occurrence of local cell wall thickenings-predominantly in the upper and lower epidermis-and the symptoms of accelerated leaf senescence. Nevertheless, many chloroplasts showed almost intact chloroplast ultrastructure. Modifications in leaf anatomy could be a symptom of alterations in morphogenesis but may also be related to plant adaptation to water deficit stress. The occurrence of local cell wall thickenings can be considered as a symptom of a defence strategy involved in the enlargement of apoplast volume for toxic elements (TE) sequestration and the alleviation of oxidative stress. Importantly, the ultrastructure of leaf cells was not markedly disturbed. The results suggested that Norway maple may have good phytoremediation potential. However, the general shape of the plant, the significantly smaller size of leaves, and accelerated senescence indicated the high toxicity of the mining sludge used in this experiment. Hence, the phytoremediation of such a substrate, specifically including use of Norway maple, should be preceded by some amendments-which are highly recommended.

The efficient applications of native flora for phytorestoration of mine tailings: a pan-global survey

Mine tailings are the discarded materials resulting from mining processes after minerals have been extracted. They consist of leftover mineral fragments, excavated land masses, and disrupted ecosystems. The uncontrolled handling or discharge of tailings from abandoned mine lands (AMLs) poses a threat to the surrounding environment. Numerous untreated mine tailings have been abandoned globally, necessitating immediate reclamation and restoration efforts. The limited feasibility of conventional reclamation methods, such as cost and acceptability, presents challenges in reclaiming tailings around AMLs. This study focuses on phytorestoration as a sustainable method for treating mine tailings. Phytorestoration utilizes existing native plants on the mine sites while applying advanced principles of environmental biotechnology. These approaches can remediate toxic elements and simultaneously improve soil quality. The current study provides a global overview of phytorestoration methods, emphasizing the specifics of mine tailings and the research on native plant species to enhance restoration ecosystem services.

Past, present and future trends in the remediation of heavy-metal contaminated soil - Remediation techniques applied in real soil-contamination events

Most worldwide policy frameworks, including the United Nations Sustainable Development Goals, highlight soil as a key non-renewable natural resource which should be rigorously preserved to achieve long-term global sustainability. Although some soil is naturally enriched with heavy metals (HMs), a series of anthropogenic activities are known to contribute to their redistribution, which may entail potentially harmful environmental and/or human health effects if certain concentrations are exceeded. If this occurs, the implementation of rehabilitation strategies is highly recommended. Although there are many publications dealing with the elimination of HMs using different methodologies, most of those works have been done in laboratories and there are not many comprehensive reviews about the results obtained under field conditions. Throughout this review, we examine the different methodologies that have been used in real scenarios and, based on representative case studies, we present the evolution and outcomes of the remediation strategies applied in real soil-contamination events where legacies of past metal mining activities or mine spills have posed a serious threat for soil conservation. So far, the best efficiencies at field-scale have been reported when using combined strategies such as physical containment and assisted-phytoremediation. We have also introduced the emerging problem of the heavy metal contamination of agricultural soils and the different strategies implemented to tackle this problem. Although remediation techniques used in real scenarios have not changed much in the last decades, there are also encouraging facts for the advances in this field. Thus, a growing number of mining companies publicise in their webpages their soil remediation strategies and efforts; moreover, the number of scientific publications about innovative highly-efficient and environmental-friendly methods is also increasing. In any case, better cooperation between scientists and other soil-related stakeholders is still required to improve remediation performance.

Trace elements in Foodstuffs from the Mediterranean Basin-Occurrence, Risk Assessment, Regulations, and Prevention strategies: A review

Trace elements (TEs) are chemical compounds that naturally occur in the earth's crust and in living organisms at low concentrations. Anthropogenic activities can significantly increase the level of TEs in the environment and finally enter the food chain. Toxic TEs like cadmium, lead, arsenic, and mercury have no positive role in a biological system and can cause harmful effects on human health. Ingestion of contaminated food is a typical route of TEs intake by humans. Recent data about the occurrence of TEs in food available in the Mediterranean countries are considered in this review. Analytical methods are also discussed. Furthermore, a discussion of existing international agency regulations will be given. The risk associated with the dietary intake of TEs was estimated by considering consumer exposure and threshold values such as Benchmark dose lower confidence limit and provisional tolerable weekly intake established by the European Food Safety Authority and the Joint FAO/WHO Expert Committee on Food Additives, respectively. Finally, several remediation approaches to minimize TE contamination in foodstuffs were discussed including chemical, biological, biotechnological, and nanotechnological methods. The results of this study proved the occurrence of TEs contamination at high levels in vegetables and fish from some Mediterranean countries. Lead and cadmium are more abundant in foodstuffs than other toxic trace elements. Geographical variations in TE contamination of food crops clearly appear, with a greater risk in developing countries. There is still a need for the regular monitoring of these toxic element levels in food items to ensure consumer protection.

Ecotoxicological Assessment of Polluted Soils One Year after the Application of Different Soil Remediation Techniques

The present work evaluated the influence of eight different soil remediation techniques, based on the use of residual materials (gypsum, marble, vermicompost) on the reduction in metal(loid)s toxicity (Cu, Zn, As, Pb and Cd) in a polluted natural area. Selected remediation treatments were applied in a field exposed to real conditions and they were evaluated one year after the application. More specifically, five ecotoxicological tests were carried out using different organisms on either the solid or the aqueous (leachate) fraction of the amended soils. Likewise, the main soil properties and the total, water-soluble and bioavailable metal fractions were determined to evaluate their influence on soil toxicity. According to the toxicity bioassays performed, the response of organisms to the treatments differed depending on whether the solid or the aqueous fraction was used. Our results highlighted that the use of a single bioassay may not be sufficient as an indicator of toxicity pathways to select soil remediation methods, so that the joint determination of metal availability and ecotoxicological response will be determinant for the correct establishment of any remediation technique carried out under natural conditions. Our results indicated that, of the different treatments used, the best technique for the remediation of metal(loid)s toxicity was the addition of marble sludge with vermicompost.

The Control of Novel and Traditional Elemental Impurities: Ag, Au, Co, Cs, Li, Mo, Se, Sr, and V in Mint Tea Infusions (Peppermint, Mentha piperita L.) Available in Poland: A Health Risk Assessment

The purpose of our studies is the evaluation of the health risks of the novel elemental impurities (Ag, Au, Co, Cs, Li, Mo, Se, Sr, and V) in mint tea infusions (Mentha piperita L.) available in Poland. For this purpose, we applied ICP-MS-based methodology for content analysis (elemental profile and µg/L of the infusion) and also the specific health risk assessment for a comprehensive assessment. Our strategy was based on weekly intake (µg/L of infusion/week) considering weekly tea consumption. Then, based on the weekly consumption of tea per adult, individuals were compared to the Temporary Tolerated Weekly Intake (PTWI) designated by the Joint FAO Expert Committee on Food Additives (JECFA), and the weekly consumption per body weight (µg/L of infusion/week/bw) was estimated. Daily exposure to Co in the tested products is in a range of 0.093-0.238 µg/day. In comparison, the established PDE (oral exposure) for Co by the ICH guideline (the ICH guideline Q3D (R1), 2019) is 50 µg/day. For lithium, PDE is approximately 560 µg/day and, in our study, the estimated daily exposure for Li in investigated products is in a range of 0.226-2.41 µg/day. Our studies found also low levels of Cs (in a range of 0.0598-0.195 µg/L), Mo (in a range of 0.663-3.261 µg/L), and Sr (0.223-65.842 µg/L) in infusions. For Molybdenum, the established PDE is approximately 3400 µg/day. There are no documents about Sr and Co in the Joint FAO/WHO Expert Committee on Food Additives (JECFA) database and the ICH guideline Q3D (R1). In the case of V, the established PDE is 120 µg/day, and the level of V in analyzed infusions is in a range of 0.284-0.702 µg/L. Silver and gold were present only in a few samples, and the estimated daily exposure for Ag is approximately 0.04575 µg/day for product A and approximately 0.1885 µg/day for product O, except for Au, which is in a range of 0.000356-0.114 µg/L. The estimated daily exposure for Ag is 167 µg/day and, for Au, it is in a range of 8.9 × 10⁵-0.0285 µg/day. It can be concluded that levels of all investigated elements (novel and also traditional elemental impurities) in the daily dose should not threaten the consumer's health after consuming mint tea infusions.

Trace elements apportionment in forage, soil, and livestock in rangeland ecosystems along climatic gradients

BACKGROUND

Alpine meadows, typical steppes, and deserts are among the globally important rangeland types that are generally distributed along temperature and precipitation gradients. Mineral losses caused by grazing are one of the key factors that can lead to instability or even degradation of these rangeland ecosystems.

METHODS

We examined the concentrations of Cu, Fe, Mn, and Zn in soil, forage, and livestock dungs from diverse rangeland types in northwest China, to determine the relationships between these trace elements (TEs) concentrations and climatic factors (i.e., temperature, precipitation, and humidity), and to evaluate the potential risks of TEs deficiencies or excesses in these rangeland ecosystems.

RESULTS

Forage Zn concentrations in forage of all three types of rangeland, and Cu concentrations in forage of the alpine meadow did not meet the growth requirements of grazing livestock. Concentrations of Cu, Fe, and Mn in forage and Fe, Mn, and Zn in livestock dungs had quadratic parabola relationships with temperature, precipitation, and humidity, but the relationships between climate factors and Cu, Fe, and Mn concentrations in soil were not significant. In addition, the abilities of the plant to absorb Cu, Fe, and Zn from soil were stronger in the typical steppe than that in the alpine meadows and desert. Also, the abilities of livestock to return TEs to soil were stronger in the alpine meadow than that in the typical steppe and desert.

CONCLUSION

We derived a conceptual mode that the ratio of TE concentrations of the plant to soil and of livestock dung to forage represents the abilities of plants to absorb TEs from the soil matrix and livestock to return TEs to soil or to absorb TEs from forage, respectively. Results indicate potentially more serious risks of TEs deficiencies, especially that of Zn than previously considered in typical steppes and desert rangelands.

Thallium and potentially toxic elements distribution in pine needles, tree rings and soils around a pyrite mine and indication for environmental pollution

In this study, the distributions of thallium (Tl), and other potential toxic elements, such as Cd, Co, Cu, Pb, Sr, As, Cr, Ni, Zn, and Mn in needles, tree rings and soils of pine trees in one of the largest pyrite mining areas in the world, i.e., Yunfu, China were investigated. The results showed that pseudo-total Tl concentration of the tree rings ranged from 0.41 to 2.03 mg/kg (average: 1.12 mg/kg) during the year of 1998 to 2011. This indicates an overall obvious enrichment of Tl. Further investigation of element level variations in the pine needles showed a negative correlation between Tl content and the distance from the mining area. The results of Principal Component Analysis additionally demonstrated that Tl in the tree rings was most likely derived from the pine needles. Notably, Tl contents in the tree rings exhibited generally similar distribution pattern to the annual production intensity of Yunfu pyrite mining activities. The findings suggest that metal(loid)s in particular of Tl in pine tree rings can be used as alternative proxies to approximatively reconstruct the chronological change of atmospheric environmental pollution induced by pyrite associated mining/smelting activities.

Metal Accumulation and Biomass Production in Young Afforestations Established on Soil Contaminated by Heavy Metals

The restoration of forest ecosystems on metal-contaminated sites can be achieved whilst producing valuable plant biomass. Here, we investigated the metal accumulation and biomass production of young afforestations on contaminated plots by simulating brownfield site conditions. On 16 3-m² plots, the 15 cm topsoil was experimentally contaminated with Zn/Cu/Pb/Cd = 2854/588/103/9.2 mg kg⁻¹ using smelter filter dust, while 16 uncontaminated plots (Zn/Cu/Pb/Cd = 97/28/37/< 1) were used as controls. Both the calcareous (pH 7.4) and acidic (pH 4.2) subsoils remained uncontaminated. The afforestations consisted of groups of conifers, deciduous trees, and understorey plants. During the four years of cultivation, 2254/86/0.35/10 mg m⁻² Zn/Cu/Pb/Cd were extracted from the contaminated soils and transferred to the aboveground parts of the plants (1279/72/0.06/5.5 mg m⁻² in the controls). These extractions represented 3/2/3% of the soluble soil Zn/Cu/Cd fractions. The conifers showed 4–8 times lower root-to-shoot translocation of Cu and Zn than the deciduous trees. The contamination did not affect the biomass of the understorey plants and reduced that of the trees by 23% at most. Hence, we conclude that the afforestation of brown field sites with local tree species is an interesting option for their reclamation from an ecological as well as economic perspective.

Impact of Biochar Amendment on Soil Properties and Organic Matter Composition in Trace Element-Contaminated Soil

The application of biochar as an organic amendment in polluted soils can facilitate their recovery by reducing the availability of contaminants. In the present work, the effect of biochar application to acid soils contaminated by heavy metal spillage is studied to assess its effect on the quantity and composition of soil organic matter (SOM), with special attention given to soil humic acids (HAs). This effect is poorly known and of great importance, as HA is one of the most active components of SOM. The field experiment was carried out in 12 field plots of fluvisols, with moderate and high contamination by trace elements (called MAS and AS, respectively), that are located in the Guadiamar Green Corridor (SW Spain), which were amended with 8 Mg·ha⁻¹ of olive pit biochar (OB) and rice husk biochar (RB). The results indicate that 22 months after biochar application, a noticeable increase in soil water holding capacity, total organic carbon content, and soil pH were observed. The amounts of oxidisable carbon (C) and extracted HAs in the soils were not altered due to biochar addition. Thermogravimetric analyses of HAs showed an increase in the abundance of the most thermostable OM fraction of the MAS (375–650 °C), whereas the HAs of AS were enriched in the intermediate fraction (200–375 °C). Spectroscopic and chromatographic analyses indicate that the addition of biochar did not alter the composition of the organic fraction of HAs, while Cu, Fe, and as were considerably accumulated at HAs.

Long-term immobilization of soil metalloids under simulated aging: Experimental and modeling approach

Aging is an inevitable natural process, leading to faded performances of soil amendments. Understanding long-term aging features is crucial for the risk management of contaminated soil. In this study, a novel quantitative aging method, namely, the "soil coin" method, was developed, which can simulate the effects of natural aging on metal(loid) immobilization performances. To better depict the aging features, two models on the basis of conditional probability-induced failure were developed. To effectively immobilize soil arsenic (As) and antimony (Sb), magnesium (Mg) and iron (Fe) oxides were simultaneously introduced to either fresh or pre-oxidized biochar via a facile method. Although post-application aging is harmful, pre-aging (i.e., pre-oxidation using H₂O₂) in turn served as an effective means to introduce more metal oxides, thereby rendering better short-term and long-term effectiveness for metalloid immobilization. Experimental and modeling approaches suggested that precipitation accounted for long-term immobilization, while a constant aging rate is the key feature for a promising soil amendment. It is suggested that to further calibrate this method and better understand the immobilization performances in the long run, more evidence from the field is needed.

Potentially Toxic Elements’ Contamination of Soils Affected by Mining Activities in the Portuguese Sector of the Iberian Pyrite Belt and Optional Remediation Actions: A Review

Both sectors of the Iberian Pyrite Belt, Portuguese and Spanish, have been exploited since ancient times, but more intensively during and after the second half of the 19th century. Large volumes of polymetallic sulfide ore were extracted in open pits or in underground works, processed without environmental concerns, and the generated waste rocks and tailings were simply deposited in the area. Many of these mining sites were abandoned for years under the action of erosive agents, leading to the spread of trace elements and the contamination of soils, waters and sediments. Some of these mine sites have been submitted to rehabilitation actions, mostly using constructive techniques to dig and contain the contaminated tailings and other waste materials, but the remaining soil still needs to be treated with the best available techniques to recover its ecosystem functions. Besides the degraded physical structure and poor nutritional status of these soils, they have common characteristics, as a consequence of the pyrite oxidation and acid drainage produced, such as a high concentration of trace elements and low pH, which must be considered in the remediation plans. This manuscript aims to review the results from studies which have already covered these topics in the Iberian Pyrite Belt, especially in its Portuguese sector, considering: (i) soils’ physicochemical characteristics; (ii) potentially toxic trace elements’ concentration; and (iii) sustainable remediation technologies to cope with this type of soil contamination. Phytostabilization, after the amelioration of the soil’s properties with organic and inorganic amendments, was investigated at the lab and field scale by several authors, and their results were also considered.

Eurasian otter (Lutra lutra) as sentinel species for the long-term biomonitoring of the Guadiamar River after the Aznalcóllar mine spill

Accidents at mines involving stored tailings have produced catastrophic environmental damage. In April 1998 the dam of the Aznalcóllar mine tailings pond in the surroundings of the Doñana National Park (southwestern Spain) broke, discharging into the Guadiamar River more than 6 million m³ of toxic mud and acidic water with high concentrations of heavy metals and arsenic. We used the Eurasian otter (Lutra lutra) as sentinel species to assess the potential impact of the toxic spill on the river ecosystems and their recovery with time by studying the spatial and temporal variation (1999-2003, 2006) of selected trace element (Cu, Zn, Cd, Pb and As) concentrations in feces. Throughout the sampling period, the highest heavy metal and As levels were found in the most spill-affected reaches of the Guadiamar River (i.e., the Middle and, to a lesser extent, the Lower reaches), pointing out the mining accident as the main origin of the contamination. Overall, levels of trace elements decreased with the time elapsed since the toxic spill, except for Cd (F_1,352 = 0.29, P = 0.59). However, rebounds for some elements (Pb, As, and Cu) were also observed, especially in the Middle and Lower reaches of the river, which might be attributed to the residual contamination in abiotic compartments and/or new inputs from industrial and agricultural activities in the nearby areas. Concentrations were relatively high when compared to those reported for both our reference area (Guadalete River) and other metal-polluted zones. We found that the estimated amounts of Pb and As ingested during the first years after the spill in the Guadiamar Middle reach would be high enough to cause reproductive issues. This could affect the local population recovery, although evidence on distribution range and numbers suggests otherwise, with thriving populations at regional scale. Our results support the role of otters as sentinel species for biomonitoring contaminants and thus to evaluate fluvial ecosystem health.

Biochar ageing in polluted soils and trace elements immobilisation in a 2-year field experiment

Biochar application to soils has become a focus of research during the last decade due to its high potential for C sequestration. Nevertheless, there is no exhaustive information on the long-term effects of biochar application in soils contaminated with trace elements. In this work, a 2-year field experiment was conducted comprising the application of different types of biochar to acidic and moderately acidic soils with high concentrations of As, Cu, Pb, Ba and Zn. In addition, representative samples of each biochar were buried in permeable bags that allowed the flow of water and microorganisms but not their physical interaction with soil aggregates. The biochars significantly adsorbed trace elements from polluted soils. However, given the high total concentration of these persistent trace elements in the soils, the application of biochars did not succeed in reducing the concentration of available metals (CaCl₂ extractable fraction). After 2 years of ageing under field conditions, some degradation of the biochars from olive pit, rice husk and wood were observed. This study provides novel information concerning the biochar alterations during ageing in polluted soils, as the decrease of aryl C signal observed by ¹³C nuclear magnetic resonance (NMR) spectroscopy and the presence of O-containing groups shown by Fourier Transform mid-Infrared Spectroscopy (FT-IR) in aged biochar which enhanced trace elements adsorption. Scanning electron microscopy (SEM) revealed slight changes on surface morphology of aged biochar particles.

Plant response to mycorrhizal inoculation and amendments on a contaminated soil

Understanding the combined effects of soil amendments and inoculation of mycorrhizal fungi on the response of different plant species during the phytostabilization process of trace elements contaminated soils is a challenge. This task is more difficult but more realistic when studied under field conditions. We assess the combined effects of two amendment doses and mycorrhizal inoculation on the response of saplings of two tree species planted in a contaminated field. The amendments were a mix of sugar beet lime and biosolid compost. The inoculation treatments were made with a commercial inoculum of arbuscular mycorrhizal fungi for wild olive and ectomycorrhizal fungi for stone pine. Results showed a weak or null effect of the mycorrhizal inoculation on plant growth, survival and trace element accumulation. There was a significant increase on P nutrition for stone pine, growing on non-amended conditions. Soil amendments were very effective reducing trace elements availability and their accumulation in both plant species, especially in roots. However, the effects on plant biomass were species-dependent and contrasted; low-dose amendments increased the biomass of wild olive by 33.3%, but reduced by 28% that of pine. The high doses of amendments (60 T ha^-1) produced some negative effects on plant growth and nutrition, probably related to the increase of soil salinity. Both plant species, stone pine and wild olive, have been proved to be adequate for phytostabilization of contaminated soils under Mediterranean climate, due to their drought tolerance and the low transfer of trace elements from root to shoot, thus reducing toxicity for the food web. To implement microbial-assisted phytoremediation approaches, a better understanding of the diversity and ecology of plant-associated microorganisms is needed. The use of indigenous fungi, locally adapted and tolerant to contamination, would be more suitable for phytostabilization purposes.

Elemental and Thermo-gravimetric Characterization of Trace Metals in Leaves and Soils as Bioindicators of Pollution in Kyiv City

In this study, leaf and soil samples were used as bio-monitors for different alkali and heavy metals at six different locations in Kyiv city. Using x-y plots of the inductively coupled plasma optical emission spectroscopy (ICP-OES) data measured the discrepancy level in elemental composition between the different investigated areas; the correlation between the concentrations in tree leaves and the samples from the surrounding soils were investigated. While the concentration of essential mineral elements and metals was found to be similar in several leaf and soil samples, in other samples, their concentration spread up to more than one order of magnitude. The concentration of metals was found to be higher in soil samples than in leaves. Thermo-gravimetric analysis (TGA) data helped to further characterize both types of samples. The metal removal during the incineration of the leaves was investigated by coupling a thermo-gravimetric analyzer to an inductively coupled plasma optical emission spectrometer (TGA-ICP-OES). The release of Cd, K, Na, Pb, and Zn during incineration at temperatures up to 960 °C was online monitored, and some insights were drawn about the behavior of such metals and the chemistry involved in the volatilization process.

Abandoned Mine Lands Reclamation by Plant Remediation Technologies

Abandoned mine lands (AMLs), which are considered some of the most dangerous anthropogenic activities in the world, are a source of hazards relating to potentially toxic elements (PTEs). Traditional reclamation techniques, which are expensive, time-consuming and not well accepted by the general public, cannot be used on a large scale. However, plant-based techniques have gained acceptance as an environmentally friendly alternative over the last 20 years. Plants can be used in AMLs for PTE phytoextraction, phytostabilization, and phytovolatilization. We reviewed these phytoremediation techniques, paying particular attention to the selection of appropriate plants in each case. In order to assess the suitability of plants for phytoremediation purposes, the accumulation capacity and tolerance mechanisms of PTEs was described. We also compiled a collection of interesting actual examples of AML phytoremediation. On-site studies have shown positive results in terms of soil quality improvement, reduced PTE bioavailability, and increased biodiversity. However, phytoremediation strategies need to better characterize potential plant candidates in order to improve PTE extraction and to reduce the negative impact on AMLs.

Arsenic accumulation and speciation in two cultivars of Pteris cretica L. and characterization of arsenate reductase PcACR2 and arsenite transporter PcACR3 genes in the hyperaccumulating cv. Albo-lineata

Pollution and poisoning with carcinogenic arsenic (As) is of major concern globally. Interestingly, there are ferns that can naturally tolerate remarkably high As concentrations in soils while hyperaccumulating this metalloid in their fronds. Besides Pteris vittata in which As-related traits and molecular determinants have been studied in detail, the As hyperaccumulation status has been attributed also to Pteris cretica. We thus inspected two P. cretica cultivars, Parkerii and Albo-lineata, for As hyperaccumulation traits. The cultivars were grown in soils supplemented with 20, 100, and 250 mg kg-1 of inorganic arsenate (iAsV). Unlike Parkerii, Albo-lineata was confirmed to be As tolerant and hyperaccumulating, with up to 1.3 and 6.4 g As kg-1 dry weight in roots and fronds, respectively, from soils amended with 250 mg iAsV kg-1. As speciation analyses rejected that organoarsenical species and binding with phytochelatins and other proteinaceous ligands would play any significant role in the biology of As in either cultivar. While in Parkerii, the dominating As species, particularly in roots, occurred as iAsV, in Albo-lineata the majority of the root and frond As was apparently converted to iAsIII. Parkerii markedly accumulated iAsIII in its fronds when grown on As spiked soils. Considering the roles iAsV reductase ACR2 and iAsIII transporter ACR3 may have in the handling of iAs, we isolated Albo-lineata PcACR2 and PcACR3 genes closely related to P. vittata PvACR2 and PvACR3. The gene expression analysis in Albo-lineata fronds revealed that the transcription of PcACR2 and PcACR3 was clearly As responsive (up to 6.5- and 45-times increase in transcript levels compared to control soil conditions, respectively). The tolerance and uptake assays in yeasts showed that PcACRs can complement corresponding As-sensitive mutations, indicating that PcACR2 and PcACR3 encode functional proteins that can perform, respectively, iAsV reduction and membrane iAsIII transport tasks in As-hyperaccumulating Albo-lineata.

Ecological risk assessment of trace metals in soils affected by mine tailings

Environmental impacts caused by mine dam ruptures or inappropriate tailing depositions represent a global concern. An ecological risk assessment was performed in 18 areas affected by the collapse of a major mining dam in southeastern Brazil, in two monitoring periods (2015 and 2018). In these areas, pedogeochemical surveys, and ecological risk levels were determinate. In addition, ecotoxicological assays with Proisotoma minuta (Collembola) were carried out in laboratory. Soil screening values indicated that all contaminated areas were above regional reference values for soil quality for at least one metal (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn), likewise exceeding threshold values for potential ecological and human health risks. In two monitoring years, significant ecotoxicity in the avoidance and reproduction of P. minuta (> 60 % and >80 %, respectively) were evidenced in most soils; and lethal responses in some areas like Córrego Novo, Governador Valadares and Tumiritinga. Results suggest changes in soil physical-chemical properties due to tailing deposition, thus affecting soil dwellers. This study can elucidate the use of appropriate tools to ecological risk assessments, helping to identify the priority areas for defining remediation and monitoring strategies.

Long-term trace element assessment after a mine spill: Pollution persistence and bioaccumulation in the trophic web

Trace elements can be toxic when they cannot be easily removed after entering an ecosystem, so a long-term assessment is fundamental to guide ecosystem restoration after catastrophic pollution. In 1998, a pyrite mining accident in Aznalcóllar (south-western Spain) spilled toxic waste over a large area of the Guadiamar river basin, where, after restoration tasks, the Guadiamar Green Corridor was established. Eight years after the mine accident (2005-2006), the ground-dwelling insectivorous lizard Psammodromus algirus registered high trace-element levels within the study area compared to specimens from a nearby unpolluted control site. In 2017, 20 years after the accident, we repeated the sampling for this lizard species and also quantified trace elements in vegetation as well as in arthropod samples in order to identify remnant trace-element accumulation with the aim of assessing the transfer of these elements through the trophic web. We found remnant trace-element contamination in organisms of the polluted site compared to those from the unpolluted site. All trace-element concentrations were higher in arthropods than in plants, suggesting these compounds bioaccumulate through the trophic web. Lizards from the polluted areas had higher As, Cd, and Hg concentrations than did individuals from the unpolluted area. Lizard abundance between sampling periods (2005-06 and 2017) did not vary in unpolluted transects but strongly declined at polluted ones. By contrast, the Normalized Difference Vegetation Index indicated that in the study period, the vegetation was similar at the two sampling sites. These results suggest that, 20 years after the accident, the trace-element pollution could be the cause of a severe demographic decline of the lizard in the polluted area.

Land Use in Habitats Affects Metal Concentrations in Wild Lizards Around a Former Lead Mining Site

We investigated the potential effects of different land use and other environmental factors on animals living in a contaminated environment. The study site in Kabwe, Zambia, is currently undergoing urban expansion, while lead contamination from former mining activities is still prevalent. We focused on a habitat generalist lizards (Trachylepis wahlbergii). The livers, lungs, blood, and stomach contents of 224 lizards were analyzed for their lead, zinc, cadmium, copper, nickel, and arsenic concentrations. Habitat types were categorized based on vegetation data obtained from satellite images. Multiple regression analysis revealed that land use categories of habitats and three other factors significantly affected lead concentrations in the lizards. Further investigation suggested that the lead concentrations in lizards living in bare fields were higher than expected based on the distance from the contaminant source, while those in lizards living in green fields were lower than expected. In addition, the lead concentration of lungs was higher than that of the liver in 19% of the lizards, implying direct exposure to lead via dust inhalation besides digestive exposure. Since vegetation reduces the production of dust from surface soil, it is plausible that dust from the mine is one of the contamination sources and that vegetation can reduce exposure to this.

Coupling of the AQUATOX and EFDC Models for Ecological Impact Assessment of Chemical Spill Scenarios in the Jeonju River, Korea

In this study, an ecological impact was assessed for the short-term leak scenario through the AQUATOX-EFDC model, which combines the proven ecological model AQUATOX with the hydrodynamic model EFDC. A case study of the coupled AQUATOX-EFDC model was conducted for 30-30,000 kg toluene leak scenarios in the Jeonju River in South Korea. A 21-day scenario simulation was conducted, and the impact of the toluene spill accident was evaluated by comparing the biomass between the control simulation and the perturbed simulation. As a result of the simulation, it was found that in the scenario in which 3000 kg of toluene was leaked for a day, a substantial change was expected in the range of 0-640 m from the accident site. Additionally, for a 30,000 kg leak, a substantial change was expected in the range of 0-2300 m from the accident site, and the greatest damage was observed for the fish species group, the top predators. As a result, the AQUATOX-EFDC simulation showed a significant ecological impact, and the proposed model will be helpful to understand the ecological impact and establish the management strategy for the ecological risk of the chemical spill.

Assessment and monitoring of soil and plant contamination with trace elements around Europe's largest copper ore tailings impoundment

Europe's largest copper ore tailings impoundment has been considered a potential source of risk for human health, thus leading to the elimination of agricultural production in the surrounding area and its subsequent afforestation. The aim of this study was to analyse the level, spatial distribution and temporal changes in soil and edible plant contamination with trace elements around the impoundment, taking into account the local soil properties. The mean concentrations of Zn, Pb and As (31.5, 19.3 and 3.9 mg kg^-1, respectively) were found to be higher than median values in soils of Poland, but they do not indicate soil pollution or enrichment when assessed using the relative geochemical index (I_geo) and enrichment factor (EF). The Cu concentration (mean 25.6 mg kg^-1) was significantly higher than the median value for Polish soils, indicating moderate to high pollution/enrichment. A relationship between Cu concentration in topsoil and distance to the impoundment, in particular in its eastern forefield, indicates that this landfill site may be considered the source of soil contamination with Cu. However, both the mean and maximum concentrations of all elements under study, including Cu, were below the legal intervention levels. Long-term topsoil monitoring, although high data variability on permanent plots, has documented stable concentration or slow decrease of element concentration over a period 1995-2016. Cu and Zn concentrations in vegetables from home gardens do not differ from typical values in commercially available products, whereas higher than typical concentrations of Pb and As may result from other local sources of contamination. Low level and stable soil and plant contamination with trace metals justifies continuation of crop production and no need for the further conversion of arable lands into forests. The relatively little negative impact of the tailings impoundment, despite its large dimensions, results probably from implemented effective anti-emission measures.

Influence of Mining and Vegetation Restoration on Soil Properties in the Eastern Margin of the Qinghai-Tibet Plateau

Mining causes serious destruction of the surface morphology and soil structure of lands, and vegetation restoration on post-mining lands provides an effective way for soil and water conservation. To determine the influence of mining and vegetation restoration on soil properties in the eastern margin of the Qinghai-Tibet Plateau, four land sites, including two vegetation restoration sites (restorated by Elymus nutans and Picea crassifolia, respectively), one non-vegetated mining site and one native grassland site, were selected. Fifty-two topsoil (0–10) samples were collected from these four sites, and then soil properties, trace metals and soil enzyme activities were analyzed. The results showed that there was an increase in soil pH (>8.0) after mining, while vegetation restoration decreased the soil pH compared with native grassland; the soil organic matter and total nitrogen in the site restored with E. nutans increased by 48.8% and 25.17%, respectively, compared with the site restored with P. crassifolia. The soil enzyme activities decreased after mining, and there were no significant increases in urease, phosphatase, β-glucosidase and β-1,4-N-acetylglucosaminidase activities after five years of restoration. In addition, the contents of soil trace metals (cadmium, chromium, mercury, lead and zinc) after mining were lower than the Chinese threshold (GB 15618/2018), but the content of arsenic in non-vegetated soil and P. crassifolia-restored soil exceeded the threshold by 22.61 times and 22.86 times, respectively. Therefore, As-contaminated land areas should be accurately determined and treated in a timely way to prevent arsenic from spreading, and plant species with tolerance to alkaline soil should be selected for vegetation restoration on post-mining lands.

Distribution of metals in sediments of the Guadiamar river basin 20 years after the Aznalcóllar mine spill: Bioavailability and risk assessment

In April 1998, 6 million m³ of acid water and sludge were accidently spilled on to the Guadiamar riverbed (South of Spain). In this study, the long term distribution of Al, Cd, Cu, Fe, Mn, Pb, and Zn in the sediments of the Guadiamar river basin is described using fractionation analysis. Changes in the availability of metals from 2002 to 2018, covering a period of 20 years after the dam collapse, have been evaluated and their potential environmental risks have been examined. A substantial decrease in the concentrations of all the metals studied was observed, except Pb. However, the concentrations of Cu, Zn, and Pb remained higher than the background concentrations, which indicates a high influence of the mining activity, even in 2018. An increase of the residual fraction was observed from 2002 (47%) to 2018 (67%), which was mainly due to the mobilisation of metals from their oxidisable fraction to their residual fraction. Environmental risk assessment revealed a significant decrease in the risk associated with metals from 2002 to 2018, mainly due to the decrease of the metals concentration in the sediments over the year and to the lower availability of these metals in 2018. In 2002, the most challenging metals were Pb and Zn, whereas in 2018, Zn and Cd were the most problematic due to their toxicity and availability.

Accumulation of heavy metals in phytoliths from reeds growing on mining environments in Southern Europe

In Southern Europe, soil contamination by heavy metals (HM) due to mining and industrial activities is a long-known problem. Yet, despite soils being widely contaminated through decades, some plants are still able to grow. Some of these plants, like giant reed (Arundo donax) or common reed (Phragmites australis) are capable of accumulating substantial amount of HM. These reeds also contain small silica structures in their shoots, called phytoliths. However, the role of phytoliths in reducing stress caused by these HM remains unknown. The aim of this work is then to determine if phytoliths represent a preferential structure for the bioacccumulation of HM in plants. Therefore, plants from mining-contaminated sites in Spain and France were sampled and HM concentrations in total plant shoots were compared to those in phytoliths for eight metal(oid)s: As, Cd, Cu, Mn, Pb, Sb, Sn and Zn. Results show that Arundo donax and Phragmites australis tend to accumulate Cd, Sb and Sn but limit the uptake of As, Cu, Mn, Pb and Zn in plant shoots despite that the concentration of these HM in soil is quite high. Therefore, reeds appear as tolerant to high metal concentrations in soils, and phytoliths are identified as preferential structures for encapsulation of As, Cu, Mn, Pb and Zn, while Cd, Sb and Sn were mainly found to be accumulated in organic tissues rather than in phytoliths.

Copper stress in flooded soil: Impact on enzyme activities, microbial community composition and diversity in the rhizosphere of Salix integra

Climate change has increased flooding frequency, making the heavy metal polluted areas more vulnerable, and led to increased global land degradation. Information about the alteration of soil microbiota under heavy metal pollution and flooding is still rather limited. Fast-growing trees are candidates for phytoremediation of heavy metal polluted soils. Therefore, the impact of Cu pollution on microbiota in soil used for cultivating Salix integra Thunb. was investigated with and without flooding for 60 d. Bacterial and fungal communities were accessed via partial 16S rRNA (V₃-V₄) and internal transcribed spacer (ITS) genes. The activity of invertase, urease and cellulase were markedly decreased by 28.5-59%, 55.0-76.7% and 17.3-34.1%, respectively, with increasing Cu levels. Flooding significantly increased the activity of polyphenol oxidase and peroxidase by 56.3% and 41.4% at the highest Cu level compared to its respective non-flooded condition. High Cu concentration significantly decreased the richness and diversity of the bacterial community, and fungi were more sensitive than bacteria under flooding conditions. Redundancy analysis suggests that Cu, Fe and soil organic matter are the key determinants affecting the composition of microbial communities. Our findings provide new insight into the responses of soil microbes to Cu-contamination and contribute to our understanding of metal toxicity in soil-woody plant systems under flooded conditions.

Evolution of the Residual Pollution in Soils after Bioremediation Treatments

This study is focused on one of the most important spills that ever happened in Europe, the Aznalcóllar’s mine spill. The extensive application of phytostabilization in the area led to the implementation of the Guadiamar Green Corridor (GGC). Soil physicochemical properties were analyzed and the total concentration of Pb, As, Zn and Cu was measured by X-ray fluorescence (XRF); bioassay using Lactuca sativa L. was applied to assess potential toxicity. Two decades after the accident, some soils affected by residual contamination continue to appear in the area. According to regulatory levels, Pb and As concentrations are exceeded in around 13%and 70%, respectively, in the uppermost part of the soils (first 10 cm). The change in soil properties after bioremediation treatment positively promoted the reduction in the potential mobility of pollutants by the increase in pH, CaCO3 content and organic carbon. Anyway, the bioassay with Lactuca sativa, indicated that around 25% of the soils showed toxicity by the reduction of the root elongation in relation to the control samples. Our results indicate that monitoring of the GGC is still needed, together with the application of soil recovery measures to reduce the potential toxicity in some sectors of the affected area.

The role of organic amendment in soils affected by residual pollution of potentially harmful elements

The addition of organic amendment in soils affected by residual pollution of potentially harmful elements (PHEs) is evaluated. The area was polluted twenty years ago and remediation actions were intensively applied, but evidence of pollution are still detected in some sectors. The amendment application produces significant changes in the main soil properties and modifies the mobility and availability of the pollutants. In general, Cu, Zn, Cd and Pb, showed a significant reduction in soluble and exchangeable forms after the vermicompost addition (percentage of reduction ranging from 59% for soluble Pb to 95% for exchangeable Zn), both in highly (UVS) as in moderately (VS1) polluted soils. This reduction is strongly related to the rise in OC content and pH. Arsenic presented no significant reduction or even an increase in soluble forms in moderately polluted soils (VS1), where the competing effects of OC and phosphorous could be responsible for this increase. Pb also showed an increase in availability after vermicompost application, probably related to the competing effect of Mg²⁺ coming from the organic amendment. The less mobile forms (those extracted with oxalic-oxalate, pyrophosphate and EDTA), indicate that vermicompost application reduce medium-long term mobility to similar values of those found in less polluted soils (VS2); anyway, an increase in available forms of Pb and As was detected in some cases, indicating a potential risk of toxicity that should be monitored over time.

Implications of spatial heterogeneity of tailing material and time scale of vegetation growth processes for the design of phytostabilisation

Phytostabilisation projects for tailing dams depend on processes occurring at spatial scales of 10⁶ m² and at decadal time scales. Most experiments supporting the design and monitoring of such projects have much smaller spatial and time scales. Usually, they are only designed for one single scale. Here, we report the results of three coupled experiments performed at pot, lysimeter and field plot scales using six sampling periodstimes from 3 to 20 months. The work explicitly accounts for the sampling times when evaluating the effects of amendments on the performance of plants grown in tailing substrates. Two treatments with potentially complementary roles were applied: zeolites to decrease availability of Cd, Cu, Pb and Zn and green fertilizer to increase the availability of nutrients. Zeolites have a positive influence on plant development, especially in the early stages. Analyses of the pooled datasets for all sampling times revealed the possibility of predicting plant physiological variables, such as protein concentrations, pigments and oxidative stress enzyme activities, as a function of the factors extracted by principal component analysis from the metal concentrations in plants, phosphorus concentrations in plants, and sampling times. Two potentially general methodological rules were extracted: account for the spatial geochemical variability of tailings, and cover the broadest possible range of time scales by experiments. The proposed experimental methodology can be of general use for the design of tailing dam remediation technologies with improvements involving the set of measured variables and sampling frequency and by carefully relating the costs to the institutional aspects of tailing dam management.

Zirconium as a suitable reference element for estimating potentially toxic element enrichment in treated wastewater discharge vicinity

The suitability of a reference element or normalizer used in assessing soil contamination levels using enrichment factor (EF) is important for soil quality assessment and monitoring. This study evaluated the results of using three reference elements Ti, Fe, and Zr for EF determination of Rb and Sr in soils within treated wastewater discharge vicinity, Central Botswana. The upper continental crust (UCC), world average values (WAV), and the local background values (LBV) were used in EF assessment of eight pedons. The elemental concentrations of the soils were determined with portable X-ray fluorescence (pXRF) analyzer. Relationships between the elements were strongly significant between Rb and Ti (r = 0.600, p < 0.01), Rb and Fe (r = 0.735, p < 0.01), Sr and Ti (r = 0.545, p < 0.01), and Sr and Fe (r = 0.841, p < 0.01). Second-level correlation analysis between contamination factor (CF) and EF levels showed Zr as the best reference element for Rb and Sr in the soils. Results from this study provide baseline knowledge necessary for contamination assessment and monitoring of soils with similar environmental conditions.

Impact of poplar-based phytomanagement on metal bioavailability in low-phosphorus calcareous soil with multi-metal contamination

Bioavailability of trace metals (TMs) is the key component in the management of TM-contaminated soils; however, its impact mechanism is unclear in low-phosphorus (P) calcareous soils afforested by fast-growing tree species for a long duration (>10 years). We selected a site contaminated with multiple TMs and phytomanaged by poplar (Populus hopeiensis Hu & Chow) to study the impact mechanism of plant-soil interactions on TM bioavailability along a long-term chronosequence (i.e., 10, 15, 20, and 25 years). We found that phytomanagement significantly decreased soil organic carbon (SOC) content, soil total nitrogen (N) content, and soil C/P and N/P ratios with stand age, but did not significantly change soil total P content. In contrast, soil available P content significantly changed in rhizospheric soils compared with the bulk soil, suggesting the tight coupling between the amplification of P turnover and N availability. Soil pH in rhizospheric soils significantly decreased by 0.22 to 0.32 units, while calcium carbonate (CaCO₃) content decreased by 14% to 39%, as compared with the bulk soil. Bioavailable concentrations of cadmium, lead, and zinc were positively correlated with soil available P, whereas bioavailable nickel concentration was negatively correlated with soil pH. Furthermore, TM bioavailability in rhizospheric soils significantly increased with stand age, regardless of the metal type. Our results suggest that P mobilization associated with SOC depletion induced soil acidification followed by CaCO₃ dissolution, collectively leading to metal mobilization with stand age.

Cadmium excretion via leaf hydathodes in tall fescue and its phytoremediation potential

Cadmium (Cd) contamination of the soil is one of the most serious environmental problems of agricultural production. Phytoremediation has attracted increasing attention because it can safely remove the soil contaminates via plant uptake, accumulations and plant harvesting. However, the high Cd toxicity to plant tissues and treatment of the large amount of hazardous plant residues from phytoremediation have limited its commercial implementation. Here we show that the leaves of the tall fescue (Festuca arundinacea) can excrete Cd out to avoid Cd toxicity in plant tissues. Cd specific fluorescence spectroscopy with laser confocal scanning microscope, screening electron microscope with energy dispersive spectroscopy and guttation fluids analysis confirmed that leaf hydathodes were the pathway of Cd excretion in tall fescue. Element analysis showed that Cd was preferentially excreted out when compared to the ion nutrients. The amount of leaf Cd excretion was linearly increased in response to the Cd stress period. The phytoremediation efficiency was evaluated to remove 14.4% of soil Cd annually by the leaf Cd excretion in our experimental system. These findings indicate that a novel strategy of Cd phytoexcretion based on washing-off and collection of leaf surface Cd is feasible to avoid Cd toxic in plant tissues and the high treatment cost of hazardous plant residues.

Municipal solid wastes as a resource for environmental recovery: Impact of water treatment residuals and compost on the microbial and biochemical features of As and trace metal-polluted soils

In this study we evaluated the microbiological and biochemical impact of iron-based water treatment residuals (Fe-WTRs) and municipal solid waste compost (MSWC), alone and combined, on three different soils co-contaminated with arsenic (As) and trace-metals (TM), i.e. Pb, Cu and Zn. Overall, all the amendments considered significantly increased the abundance of culturable heterotrophic bacteria, with MSWC showing the greatest impact across all soils (up to a 24% increase). In most of treated soils this was accompanied by a significant reduction of both the (culturable) fungal/bacterial ratio, and the proportion of culturable As(V)- and As(III)-resistant bacteria with respect to total bacterial population. The catabolic potential and versatility of the resident microbial communities (assessed by community level physiological profile) was highly soil-dependent and substantial increases of both parameters were observed in the amended soils with the higher total As concentration (from approx. 749 to 22,600 mg kg^-1). Moreover, both carbon source utilisation profile and 16S rRNA soil metagenome sequencing indicated a significant impact of MSWC and Fe-WTRs on the structure and diversity of soil microbial communities, with Proteobacteria, Actinobacteria and Firmicutes being the most affected taxa. The assessment of selected soil enzyme activities (dehydrogenase, urease and β-glucosidase) indicated an increase of metabolic functioning especially in soils treated with MSWC (e.g. dehydrogenase activity increased up to 19.5-fold in the most contaminated soil treated with MSWC). Finally, the microbial and biochemical features of treated (and untreated) contaminated soils (i.e. total bacterial counts, catabolic potential and versatility and soil enzyme activities) were highly correlated with the concentrations of labile As and TM in these latter soils and supported a clear role of the tested amendments (especially MSWC) as As- and TM-immobilising agents.

Alterations of root architecture and cell wall modifications in Tilia cordata Miller (Linden) growing on mining sludge

Trees are considered good candidates for phytoremediation of soils contaminated with trace elements (TE), e.g. mine tailings. Using two year-old Tilia cordata plants, we demonstrated the nature and the scale of root architecture, especially root apices, as an indicator of mining sludge toxicity and plant capability to cope with these stress conditions. The novelty of our research is the analysis of the root response to substrate with extremely high concentrations of numerous toxic TE, and the 3D illustration of the disorders in root apex architecture using a clarity technique for confocal microscopy. The analysis demonstrates (1) a marked reduction in the size of the root apex zones (2) the occurrence of vascular tissues abnormally close to the root apex (3) collapse of the internal tissues in many root apices. Simultaneously, at the cellular level we observed some signs of a defensive response - such as a common increase of cell wall (CW) thickness and the formation of local CW thickenings - that enlarge the CW capacity for TE sequestration. However, we also detected harmful effects. Among others, a massive deposition of TE in the middle lamella which caused major damage - probably one of the reasons why the inner tissues of the root apex often collapsed - and the formation of incomplete CWs resulting in the occurrence of extremely large cells. Moreover, many cells of the root apex exhibited degenerated protoplasts. All these alterations indicate the harsh conditions for lime growth and survival and simultaneously, the manifestation of a defensive response. The obtained results allowed us to conclude that analysis of the nature and scale of structural alterations in roots can be useful indicators of plant ability to cope with stress conditions, e.g. in prospect of using the examined plants for reclamation of soils contaminated with TE.

Soil hydraulic properties as the main driver in the establishment of biomass crops in contaminated soils

In recent years increasing attention has been given to the potential use of contaminated lands for biofuel production, because these degraded soils cannot be used for food production. To establish these crops in Mediterranean contaminated areas, where the soil quality is usually very poor, the addition of soil amendments might be necessary to improve soil productivity. In addition, the use of crops with low water demands, adapted to these particular conditions of climate and soil contamination, is a key requirement. We studied the development of Cynara cardunculus and Silybum mariamun crops (both suitable for the production of biomass for biofuel uses under a Mediterranean climate) in trace element contaminated soils under field conditions. To our knowledge, this is the first such work under these particular experimental conditions (soil contamination and field trial). Soil physical (hydraulic), chemical, and biochemical properties were monitored for one year in experimental plots, where we tested the effects of the addition of two different amendments (sugar lime and biosolid compost) on soil functioning and crop productivity. Seed germination and plant biomass production were low, although amendment addition improved both parameters. The chemical and biological indicators (enzyme activities, PLFA profiles, and soil respiration) tended to be slightly improved by the amendments, especially sugar lime. The hydraulic properties of the soil in the experimental area were very deficient, and the effect of the amendments was not enough to improve them; this was probably the main cause of the general low productivity of these rain-fed crops, as water infiltrated poorly through the root zone. To improve crop productivity under these soil conditions, certain aspects could be improved: higher doses of amendments should be applied and deeper tillage of the soil after amendment addition should be performed to facilitate water infiltration.

Biochar amendment improves crop production in problem soils: A review

Problem soils are referred to as those with poor physical, chemical, and biological properties that inhibit or prevent plant growth. These poor properties may be a result of soil formation processes but are largely due to inappropriate farming practices or anthropogenic pollution. The world has lost a third of its arable land due to erosion and pollution in the past 40 years. Thus, there is an urgent need for improving and remediating problem soils. As a novel multifunctional carbon material, biochar has been widely used as a soil amendment for improving soil quality. Previous reviews have summarized the characteristics of biochar, the interactions with various soil contaminants, and the effects on soil quality, soil productivity, and carbon sequestration. Relatively limited attention has been focused on the effects of biochar amendment on plant growth in problem soils. As a result, a comprehensive review of literature in the Web of Science was conducted with a focus on the effects of biochar amendment on plant growth in problems soils. The review is intended to present an overview about problem soils, biochars as functional materials for soil amendment, how amended biochars interact with soils, soil microbes, and plant roots in remediation of problem soil and improve plant growth. Additionally, existing knowledge gaps and future directions are discussed. Information gathered from this review suggests that biochar amendment is a viable way of improving the quality of problem soils and enhancing crop production. It is anticipated that further research on biochar amendment will increase our understanding on the interactions of biochar with components of problem soils, speed up our effort on soil remediation, and improve crop production in problem soils.

Residual pollution and vegetation distribution in amended soils 20 years after a pyrite mine tailings spill (Aznalcóllar, Spain)

The present work assesses the residual pollution in the Guadiamar Green Corridor (SW, Spain) after a long-term aging process (18 years) since the accident of the Aznalcóllar pyrite mine. We have focused on the study of trace elements (Cu, Zn, Cd, As and Pb) in soils, their fractionation and the transference to the surrounding vegetation. The residual polluted areas are characterized by scattered plots with absence of vegetation, presenting high concentrations of trace elements, acidic pH and low organic carbon content. Surrounding these polluted plots, two vegetation gradient belts are clearly identified by changes in plant cover and richness. The inhibition of plant growth in the bare soils is related to the highest mobility of soluble and exchangeable Cu, Zn and Cd forms, which significantly decrease with the distance to the polluted plots. Plant richness and cover show differences between belts; bioaccumulation of trace elements in plants also differs, with a preferential accumulation in roots. Despite the low bioavailability of As and Pb in soils, bioaccumulation factors in plants for these elements are significantly higher in belt 1 in relation to belt 2. High Cu and Cd potential toxic concentrations in aerial parts of vegetation are found, posing a risk for livestock and a potential entrance to the food-chain. On the other hand, Lamarckia aurea (L.) Moench (in belt1) and Trifolium campestre Schreb. (in belt2) were the most dominant species in severely polluted soils. Elevated concentrations of trace elements in the vegetation growing in the area indicate plant adaptation mechanisms to live in these severely polluted soils, which can be used as a good bioindicator of pollution in similar polluted areas.

Assessing the quality of potentially reclaimed mine soils: Environmental implications for the construction of a nearby water reservoir

The cementation complex of Las Viñas (SW Spain) is a partially reclaimed abandoned mine site located in the drainage basin of a water reservoir currently under construction. The aim of this investigation was to analyze these mine soils to evaluate their potential environmental impact, especially on the final reservoir water quality. Results evidence the extremely high acidity of soils (pH of 3.4 and maximum potential acidity of 47 kg CaCO₃/ton), with high concentrations of trace elements, especially As, Pb and Cu. Sequential extraction data reveal the potential release of significant quantities of Mn, Cd, Cu and other easily-soluble trace elements by rainfalls. The weathering and transport of soils to the bottom sediments of the planned reservoir could lead to the release of significant quantities of toxic trace elements to the water column if anoxic (mainly As, Sb, Cr, Ni, Cu and Pb) or oxic (mainly Hg, Pb, V, Cu and As) conditions are found in the sediments. The acidity and metals released from these soils could jeopardize the quality of the reservoir waters. Remediation measures must be therefore adopted, focused on the cleanup and liming of soils in order to promote colonization and vegetation succession, thus avoiding soil erosion and limiting metal release to the hydrosphere. This study proposes the use of different low-cost materials to improve the soil quality, limiting the metal transfer to the planned reservoir water. The information contained in this study could be of great importance in other watersheds affected by abandoned mine sites.

Evaluation of hyperaccumulation potentials to cadmium (Cd) in six ornamental species (compositae)

Phytoremediation is considered as a promising soil remediation technique. In the present study, the growth responses, cadmium (Cd) accumulation and uptake capability of six popular compositae species, namely, Taraxacum mongolicum Hand.-Mazz., Tagetes erecta L., Tagetes patula L., Zinnia elegans Jacq., Centaurea cyanus L. and Gerbera jamesonii Bolus under Cd stress were investigated. Among the six compositae species, the growth of T. erecta L. and T. patula L. improved under 10 mg kg^-1 Cd exposure in term of the total biomass and height increased along with the increased Cd concentration in soil, and the growth of the two plants had no significant differences at the high Cd concentration (100 mg kg^-1), which indicated that they have good tolerance to Cd toxicity. At the same time, the two plants have higher biomass than four other plants. Furthermore, they can accumulate Cd above 100 μg g^-1dry tissue, which is the threshold value of a Cd-hyperaccumulator, and have higher Cd uptake ability, translocation factor (TF) and bioconcentration factor (BCF) values. According to these traits, it was shown that T. erecta L. and T. patula L. had strong tolerance and accumulation capability to Cd, therefore they can become potential hyperaccumulators in phytoremediation of Cd-contaminated soils.

Ectomycorrhizal Fungal Communities and Their Functional Traits Mediate Plant-Soil Interactions in Trace Element Contaminated Soils

There is an increasing consensus that microbial communities have an important role in mediating ecosystem processes. Trait-based ecology predicts that the impact of the microbial communities on ecosystem functions will be mediated by the expression of their traits at community level. The link between the response of microbial community traits to environmental conditions and its effect on plant functioning is a gap in most current microbial ecology studies. In this study, we analyzed functional traits of ectomycorrhizal fungal species in order to understand the importance of their community assembly for the soil-plant relationships in holm oak trees (Quercus ilex subsp. ballota) growing in a gradient of exposure to anthropogenic trace element (TE) contamination after a metalliferous tailings spill. Particularly, we addressed how the ectomycorrhizal composition and morphological traits at community level mediate plant response to TE contamination and its capacity for phytoremediation. Ectomycorrhizal fungal taxonomy and functional diversity explained a high proportion of variance of tree functional traits, both in roots and leaves. Trees where ectomycorrhizal fungal communities were dominated by the abundant taxa Hebeloma cavipes and Thelephora terrestris showed a conservative root economics spectrum, while trees colonized by rare taxa presented a resource acquisition strategy. Conservative roots presented ectomycorrhizal functional traits characterized by high rhizomorphs formation and low melanization which may be driven by resource limitation. Soil-to-root transfer of TEs was explained substantially by the ectomycorrhizal fungal species composition, with the highest transfer found in trees whose roots were colonized by Hebeloma cavipes. Leaf phosphorus was related to ectomycorrhizal species composition, specifically higher leaf phosphorus was related to the root colonization by Thelephora terrestris. These findings support that ectomycorrhizal fungal community composition and their functional traits mediate plant performance in metal-contaminated soils, and have a high influence on plant capacity for phytoremediation of contaminants. The study also corroborates the overall effects of ectomycorrhizal fungi on ecosystem functioning through their mediation over the plant economics spectrum.